Vascular endothelial growth factor (VEGF) is a major inducer of angiogenesis or new blood vessel formation. This protein and members of the VEGF family play critical roles during normal embryonic vasculature development and are also associated with a number of angiogenesis related pathological conditions including cancer, rheumatoid arthritis and diabetic retinopathy. VEGF exerts its biological activity by binding to two tyrosine kinase receptors, VEGF-R1 (Flt-1) and VEGF-R2 (Flk-1/KDR). Both receptors belong to the type III tyrosine kinase family and are characterized by an extracellular domain consisting of seven immunoglobulin (Ig)-like loops, a transmembrane domain and a split kinase domain within the cytoplasmic moiety (Shibuya, 2001). Several of the biochemical characteristics of the extracellular or soluble domain of Flt-1 (sFlt-1) make it a potential antagonist of VEGF activity and therefore a potential therapeutic agent. First, sFlt-1 has a much higher affinity for VEGF than Flk-1/KDR and does not need accessory proteins for ligand binding. Second, sFlt-1 also binds many of the VEGF isoforms along with other members of the VEGF family including VEGF-B and placenta growth factor (PIGF) (Hornig et al. 1999).
For a sFlt-1-based protein therapeutic to be practical for human use, it needs to have good in vivo stability and a long circulating half-life, particularly if given chronically. Unfortunately, most recombinant proteins have relatively short residence times in circulation, on the order of hours.
Therapeutic proteins are typically, but not exclusively, administered by injection. Introduction of proteins into circulation exposes them to numerous cell types, enzymes and routes of extravasation that contribute to their rapid clearance or catabolism. The protein may be attacked by plasma proteases or bind plasma proteins. Cell binding may result in the uptake of the protein via endocytotic or pinocytotic mechanisms, with the end result being degradation by lysosomal proteases. Proteins that avoid capture by these cells may pass out of the circulation via uptake by the liver, the lymphatic system or renal glomeruli (Sheffield, 2001).
Because of these rapid clearance mechanisms, circulating concentrations of injected proteins change constantly, often by several orders of magnitude, over a 24 hr period. These fluctuations can lead to decreased efficacy and increased frequency of adverse side effects for protein therapeutics. Most protein products currently on the market require frequent injections, usually multiple times per week. This dosing regimen is painful, inconvenient for the patient, and may not provide the optimum therapeutic benefit. In the case of a chronic indication such as cancer or RA, treatment could last for years.
Covalent modification of proteins with PEG has proven to be a useful method to extend the circulating half-lives of proteins in the body (Abuchowski et al., 1984; Meyers et al., 1991; Keating et al., 1993). Several PEGylated proteins are approved for use in humans or are in human clinical trials (Harris et al., 2003). Covalent attachment of PEG to a protein increases the protein's effective size and reduces its rate of clearance from the body, presumably through interference with protein removal pathways, including kidney glomerular filtration, proteolytic degradation as well as active clearance via specific receptors.
PEGs are commercially available in several sizes and shapes, allowing the circulating half-lives of PEG-modified proteins to be tailored for individual indications through the use of different PEGs. PEGylation increases a protein's effective molecular weight more than would be expected based on the molecular weight of the PEG moiety due to the water of hydration associated with the PEG group. For example, attachment of a single 5 kDa PEG to a 36 kDa protein increases the effective molecular weight of the complex to greater than 100 kDa, as measured by size-exclusion chromatography (Fee, 2003). When administered by subcutaneous injection, PEGylated proteins are slowly absorbed from the injection site, thus avoiding the serum “spikes” seen after subcutaneous injection of an unmodified protein. This “controlled release” of the PEGylated protein results in a more constant serum level, thus prolonging or increasing the drug's pharmacologic activity while minimizing the side effects typically seen with fluctuations in the drug concentrations. Other documented in vivo benefits of PEG modification include an increase in protein solubility, enhanced stability (possibly due to protection from proteases) and a decrease in immunogenicity (Keating et al., 1993).